Gattya wimleni spec. nov. is referred to the family Halopterididae (Hydroida, Thecatae). Colonies were collected at Tuléar (SW Madagascar, Indian Ocean) on the deeper part of the lower sloping platform, at 35 to 45 metres depth on the outer slope of the great barrier reef. The specimens were studied in transmitted light and by SEM. They differed from the eight species of Gattya already known in having a hydrothecal margin provided with four large rounded teeth of equal size - one abcauline, one adcauline and two lateral, curving inwards over the aperture, the laterals being hidden behind the lateral nematothecae in profile view. Its closest affinities are with the species G. multithecata (Jarvis, 1922) and G. aglaopheniaformis (Mulder & Trebilcock, 1909). A key is given for the identification of the species. Some of the descriptive characters are discussed: nodes and joints, bithalamy, and mobility of the nematothecae.